| The experimental site is selected near the HeiShiDing Nature Reserve, Guangdong province, China. We collected the seed of 82 species in the HeiShiDing Nature Reserve and constructed artificial grassland plots with various species richness (1, 2, 4, 6, 8, 10, 15, 20, 25, 30, 35, 40). After one year, the drought and un-drought condition was exposed to plots. Some parameters, such as biomass and soil nitrogen, etc., were investigated in the plots after 2 months and 4 months of drought stress. The relationship between various components of biodiversity (species richness, dominant species, functional richness and functional composition) and ecosystem function (biomass, soil nitrogen and stability) was studied.Before the treatment of drought stress, most of communities can be classified into following seven types with biomass of dominant species which took account of 60% in the plots: Triumfetta bartramia, Urena lobata, Cynodon dactylon, Gahnia tristis, Ageratum houstonianum, A/rtemisia argyi and Ambrosia artemisiifolia. There was no dominant species in other communities. Totally, there are 33 common species distributed in these plots. They took account of 95% of the total biomass in the plots. These 33 species were classified into three functional groups with the traits of life form: annual herbage, perennial herbage and shrub-subshrub. According to the combination of species with different functional traits in the plots, we classified seven types of communities with various functional compositions: annual herbage, perennial herbage, shrub-subshrub, annual herbage + perennial herbage, annual herbage + shrub-subshrub, perennial herbage + shrub-subshrub, annual herbage + perennial herbage + shrub-subshrub.(1) Dominant species has more important effect on the biomass of plot than species richness. The change of species evenness is the main reason that dominant species has significant effect on biomass, while the competition interaction between species led the change of evenness. We also find that functional composition has significant effect on the biomass of plot, and with the continuance of drought stress the effect of functional composition was stronger. The species can be classified into three functional groups: annual herbage, perennial herbage and shrub-subshrub. Under drought stress, annual herbage is the key factor that influences the biomass change of plots with different functional composition. Path analysis showed that dominant species and functional composition had important direct-effect on the biomass of plots, but species richness and functional richness had little direct-effect. Besides, there was important indirect-effect between dominant species and functional composition, and they also had important indirect-effect on other components of biodiversity. These indirect-effects were influenced greatly by drought stress. The change of the biomass with those common species showed that various species had different roles in the same functional group. This means that the species composition of functional group has significant effect on the biomass of functional group. There was a significant difference between the plots with different functional composition for the biomass of a single species. The change of competition relationship between species is an important factor that influences species biomass.(2) Dominant species has more important effect on available nitrogen (nitrate nitrogen and ammonium nitrogen) than species richness under the stress of drought. The average concentration of available nitrogen in the plots with drought stress was significantly higher than that in the contrast plots after 4 months of drought stress. In the plots with the stress of drought, the average concentration of available nitrogen after 4 months of drought stress was significantly higher than that after 2 months of drought stress. Maybe it was related to the change of environmental factors caused by drought stress, such as soil temperature and moisture, etc.. Therefore, environmental factor is the key factor that influences soil available nitrogen. LSD analysis revealed that available nitrogen concentration between the plots with various functional composition appear no significant difference in the contrast plots. In the plots with the stress of drought, the available nitrogen concentration has significant difference. This may be the reason that under drought stress, different functional group species have different effect on microorganisms that control the mineralization of nitrogen. Our research also indicated that species richness had important effect on the soil total nitrogen either in drought or un-drought plots, but dominant species and functional composition has little effect. Contrast to soil available nitrogen, soil total nitrogen was not influenced by the environmental factors. Its concentration was relatively stable under drought stress.(3) Dominant species has significant effect on the stability of plots, while species richness has little effect. The reason is that the species evenness is low in experimental plots, and the biomass of most plots is determined by one or several dominant species. We also fund that functional composition had important effect on the stability of plots. Under drought stress, the stability of plots with various functional compositions has different trends of change. The stability also increases progressively along the hierarchy of organizational level, i.e., from species to the functional group, and then to community (plot). Variant components in the organizational level have different response mechanisms to the drought stress. (4) With the analysis of the competitive hierarchies in common 29 species, we fund that Ambrosia artemisiifolia and Gahnia tristis have the highest biomass percentage (biomass percentage is used to present the comprehensive competitive hierarchies) in drought and un-drought plots after 2 months and 4 months of drought stress. They are dominant species in competition. Contrast to un-drought plots, there were significant difference of the biomass percentage in drought plots between 2 months and 4 months of drought stress. This means that the competitive hierarchies of these species change greatly. Species mortality, which is used to present the competitive ability of species survival, was significantly positive correlated between un-drought plots and drought plots. This indicated that drought stress has no significant effect on the competitive hierarchies of species survival characteristic. The increscent of species biomass percentage, which is used to present the competitive ability of species growth, was not significantly correlated between un-drought plots and drought plots. This means that drought stress has significant effect on the competitive hierarchies of species growth characteristic. Either in the un-drought or drought plots with various gradients of species richness, both survival competition ability and growth competition ability of Sida rhombifolia, Mosla dianthera and Cassia mimosoides were the lowest. This indicated that they are competitive inferior species and their competitive hierarchies don't change. In the un-drought and drought plots with various gradients of species richness, the survival competition ability of Ambrosia artemisiifolia and Gahnia tristis were the highest. Although the growth competition ability of Ambrosia artemisiifolia and Gahnia tristis changes, while their biomass percentage was still the highest. This means they are the competitive dominant species and their competitive hierarchies don't change. However, in the un-drought and drought plots with various gradients of species richness, both survival competition ability and growth competition ability of the species that have moderate comprehensive competitive hierarchies changed greatly.(5) In the drought plots,"redundancy"species that has little effect on the biomass of plots after 2 months of drought stress has more or less effect on the biomass of plots after 4 months of drought stress. However, in the contrast plots, those"redundancy"species that had little effect on the biomass of plots after 2 months of drought stress, still no effect on the biomass after 4 months of drought stress. While in the plots with the stress of drought, we found that the species with little effect on the biomass of plots after 2 months of drought stress, had various effects after 4 months of drought stress. In particular, Xanthium sibiricum and Celosia argentea have great effect on the biomass of plots after 4 months of drought stress. Therefore,"redundancy"may be just a relative concept. The"redundancy"species which has similar function in a certain environment may be has significantly different function in a different environment.
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